Many types of fuel injection systems for internal combustion engines have been developed over the years. Common rail fuel injection systems are widely used in connection with multi-cylinder internal combustion engines. A typical common rail fuel system may include a low pressure fuel source such as a fuel tank, a high pressure pump which receives fuel from the fuel tank and increases the fuel pressure to a relatively high pressure, and a common rail connecting with the high pressure pump. The common rail serves as a source of high pressure fuel for a plurality of fuel injectors associated one with each of a plurality of cylinders. Injection of fuel at the relatively high pressure of the common rail can occur relatively precisely by electronically controlling each of the fuel injectors coupled with the common rail. The high pressure fuel pump replenishes fuel consumed via fuel injection events, and maintains the rail pressure at a desired level. Common rail systems have seen widespread success in part because they provide a relatively simple and straightforward means for providing fuel to a plurality of engine cylinders via fuel injectors, and also because common rail systems have proven to be a relatively efficient and effective way to handle relatively high fuel pressures.
Common rail fuel systems have enabled engine designs and operating methods having a number of advantages over other strategies. On the one hand, injecting fuel at the relatively high pressures attainable with a common rail can increase fuel atomization in an engine cylinder and thus improve certain factors such as combustion rate and combustion completeness. Relatively high injection pressures can also be useful in controllably injecting relatively precise quantities of fuel for a variety of purposes. To further improve upon these and other advantages, engineers continue to seek out strategies for injecting fuel at ever increasing injection pressures. While common rails have long served as an industry standard for high pressure fuel injection practices, they are not without drawbacks.
For example, containing a volume of extremely highly pressurized fuel can be sometimes difficult, requiring specialized hardware, such as seals and plumbing, which can withstand the high fuel pressures. In addition, parts subjected to extremely high pressures can have a tendency to wear relatively more quickly than parts in lower pressure environments. It can also take significant engine output energy to maintain a relatively large volume of fuel at high pressure. Relying solely upon a common rail as an engine's fuel source can ultimately impact engine efficiency.
Earlier systems are known where a cam-driven piston pressurizes fuel in a fuel injector to enable fuel injection at a relatively high pressure. These systems differ from common rail systems in that fuel pressurization takes place individually at each fuel injector, rather than relying on a common high pressure fuel source. One advantage to cam actuation is that the available power for pressurizing fuel tends to be relatively high. Hence, the pressure capabilities of certain cam actuated fuel injectors are even higher than those of conventional common rail systems. A potential drawback to cam actuation is that controllability may be less than that of common rail systems. Other systems provide two different sources of fuel to enable injection at a relatively low pressure and also injection at a relatively high pressure when desired.
Still another concept which attempts to provide both a lower injection pressure and a higher injection pressure is known from U.S. Pat. No. 5,413,076 to Koenigswieser et al. (“Koenigswieser”). In Koenigswieser, a common rail is provided which is connected with a plurality of fuel injectors. Each of the fuel injectors includes a booster piston which has an end face capable of receiving fluid pressure from the common rail. The fuel injectors in Koenigswieser can be used to inject fuel at a rail pressure, then at a relatively higher pressure via common rail actuation of the piston. While systems such as that shown in Koenigswieser may provide certain advantages, they still suffer from fluid containment and other issues.